The Independent Sacramental Movement is a loose collection of jurisdictions, parishes, clergy, and lay people who value the sacramental life of the church, expressed through apostolic succession, 7 sacraments, and the trifold ministry of bishops, priests/presbyters, and deacons. This movement exists outside of the Roman Catholic and Orthodox communions, although it generally adopts much from them and the other ancient, historic expressions of Christianity.
The ISM is made up of jurisdictions. These are usually led by a single bishop, although in some cases there may be a council of bishops. Parishes and their priests make up the jurisdiction. Typically, the parishes run their own affairs and are congregational in polity. Thus parishes can leave a jurisdiction if they so choose. Priests as well are known to sometimes leave one jurisdiction and move to another, in a process called "incardination."
There are currently hundreds of jurisdictions, most of which are quite small and none of which reach the size of a mainline Protestant denomination. The thing about these jurisdictions is that they come and go. ISM bishops generally aren't paid for their role, and their ability to keep up with things and maintain (let alone grow) their jurisdictions can vary. Also, frankly, some ISM bishops are simply unprepared for their role.
A clever way that these jurisdictions have of being able to endorse priests for military chaplaincy is through affiliation with the International Council of Community Churches (ICCC). Since the ICCC is recognized by the US military for endorsing chaplains, ISM churches that are part of the ICCC are able to endorse chaplains for service. Right now, for example, the top listing in the ICCC directory for New Jersey is the American National Catholic Church, an ISM jurisdiction.
While ISM churches seek to maintain historic apostolic succession and celebrate the same sacraments as the Roman Catholic and Orthodox communions, these tend to contest the validity of their lineages and therefore sacraments. These concerns may lead to some jurisdictions being overly defensive about their succession and sacraments, but on the whole participants in the ISM simply disagree with their critics and carry on.
For a listing of many ISM jurisdictions, check out this link.
